Various devices have been used for years in many aircraft wing designs in order to prevent the airflow from separating prematurely from the wing at high angles of attack, and thereby reduce the stall speed and improve handling at low speeds. Air from below the wing accelerates through the device and exits rearward and substantially parallel to the upper wing surface, energizing the boundary layer and delaying separation. A similar approach provides a pressurized discharge from wing surface openings, either from compressed air tanks, pumps, or from the jet engines, to accomplish a similar goal. Because most of the devices also tend to contribute to drag at cruising speeds, retractable devices may be used instead to provide adjustable modifications that can be closed when not needed. Other devices may also be used to reduce wave drag under transonic conditions to distribute pressure and increase the critical Mach number, thereby improving performance at such speeds. Variations of this basic theme of using slots, slats or other devices to ensure more attached flow across the wing surfaces and delayed boundary layer separation are described in U.S. Pat. No. 2,041,786 (Stalker); U.S. Pat. No. 2,507,611 (Pappas et al.); U.S. Pat. No. 2,571,304 (Stalker); U.S. Pat. No. 2,587,359 (Milans); U.S. Pat. No. 3,208,693 (Riedler et al.); U.S. Pat. No. 3,363,859 (Watts); U.S. Pat. No. 3,897,029 (Calderon); U.S. Pat. No. 4,641,799 (Quast et al.); U.S. Pat. No. 4,664,345 (Lurz); U.S. Pat. No. 5,255,881 (Rao); U.S. Pat. No. 5,788,190 (Siers); U.S. Pat. No. 6,293,497 (Kelley-Wickemeyer et al.); U.S. Pat. No. 6,328,265 (Dizdarevic); U.S. Pat. No. 6,905,092 (Somers); U.S. Pat. No. 7,048,235 (McLean et al.); and U.S. Patent Application Publication No. 2007/0034746 (Shmilovich et al).
Another problem in aircraft design is the formation of trailing vortices and wake turbulence during flight. Throughout the history of aeronautics, aircraft designers have had to deal with the energy consuming effects of wingtip vortices, which form at the tip of a wing where higher pressure air from beneath the wing flows in a generally span-wise direction around the wingtip to the lower pressure region above the wing. Wingtip vortices have been considered by many experts to be an essentially unavoidable consequence of a wing producing lift. These vortices are associated with lift-induced drag and are a major component of wake turbulence. Smaller vortices are induced at other points on an aircraft wherever there is an abrupt change in planform or contour, such as at the outboard tips of wing flaps, ailerons, horizontal stabilizers, elevators and other flight control surfaces. The drag and wake turbulence from the vortices have a negative impact on fuel efficiency and flight performance of the aircraft, and also pose a safety hazard to any aircraft that follow too closely or otherwise cross the wake.
Various solutions have been developed in an effort to reduce the formation of trailing vortices, especially wingtip vortices. Since wingtip vortices only affect that portion of a wing closest to its tip, one partial solution is to use a higher aspect ratio wing (longer wingspan and/or reduced chord), but this also tends to reduce aircraft maneuverability and adds structural weight. Another approach is to modify the lift distribution along the span to generate more lift at the wing root and less toward the wing tip, by modifying the wing planform and twist. However, these solutions do not make full use of the wingspan to efficiently produce lift.
A number of wingtip devices have been designed to allow nearly the entire wingspan to produce lift, while simultaneously altering the airflow near the wingtips in order to affect the vortices or to change the pattern of vorticity so as to reduce the associated drag. The intended result is reduced drag, with a corresponding improvement in fuel efficiency. Drooped (Hoerner) wingtips focus the vortex away from the upper wing surface. Winglets, a near-vertical upward or downward extension of the wingtips, cause the vortex to strike the surfaces of the winglet so as to generate an inward and slightly forward force, and thereby convert some of the vortex energy into an apparent thrust. Wingtip fences are winglet variations that may have surfaces extending both above and below the wingtip to reduce the span-wise component of airflow that leads to wingtip vortices, but create new vortices at the fence tips as well as interference drag, albeit displaced from the main wing. Blended winglets smoothly curve up, increasing their cant gradually to reduce interference drag that would otherwise occur at the wing/winglet junction. Raked wingtips have a higher degree of sweep than the rest of the wing. Various wingtip devices are described, for example, in U.S. Pat. No. 5,039,032 (Rudolf); U.S. Pat. No. 5,634,613 (McCarthy); U.S. Pat. No. 6,722,615 (Heller et al.); and U.S. Pat. No. 6,892,988 (Hugues).
In U.S. Pat. No. 5,823,480, La Roche provides a wing grid having at least two parallel staggered “winglets” extending outward from the free end (tip) of the wing. The winglets, which in this case are essentially parallel to the main wing rather than upward extending, subdivide the air circulation at the tip so that the span-wise lift distribution is more regular, decreasing induced resistance. In one embodiment, this wing grid can be retracted into the end of the wing.
In U.S. Pat. No. 4,478,380, Frakes discloses a wingtip vortex suppressor that utilizes a scoop having an inlet at a lower leading surface and an outlet at an upper trailing surface to reduce the pressure differential at the wing's trailing edge. The scoop is inboard of a turbulence fence at the wingtip. In another arrangement, U.S. Pat. No. 5,806,807 to Haney, has both a deflector extending from the top surface of the wing inboard of the tip and an air passage extending from an inlet on a high pressure side of an airfoil through the airfoil to an outlet on a low pressure side of the airfoil, with the outlet positioned between the deflector and the wing tip. The deflector and air passage work in combination to attenuate the wingtip vortex. In both patents it appears that the fence or deflector is an essential component for vortex reduction.
In U.S. Pat. No. 5,791,875, Ngo describes a system providing a source of positive fluid pressure interior to the wing and a fluid router (curved slots in lower surface of the wingtip) that directs the fluid inboard against the outward airflow in order to reduce the wingtip vortex. Similarly, in U.S. Pat. No. 7,134,631, Loth provides a tip circulation control that blows air out from the very end of the wing in order to cancel opposing vorticity. In U.S. Pat. No. 7,100,875, Shmilovich et al discharge a jet air stream from a set of nozzles at the wingtips and moves them cyclically back and forth in order to dissipate and scatter wingtip vortices. Although from a safety standpoint any reduction in trailing vortices is an improvement, from an energy efficiency standpoint the amount of energy required to sustain such counteracting airflows tends to defeat any fuel savings that might be achieved by a decrease in drag.
Trailing vortices and wake turbulence can also cause drag in other fluid dynamic structures, such as in rotor blades in helicopters and wind-power turbines, sails (which are also wings), and underwater surfaces such as hydrofoils, hulls, centerboards, keels, rudders and screws of various watercraft including submersibles. U.S. Pat. No. 5,374,013 to Bassett et al. describes one approach to reducing drag in a truck by providing a pressure shell around the rear of the truck body with better boundary flow around the truck. In any of these fluid dynamic structures, it is desired to realize fuel savings and safety improvements by reducing trailing vortices and wake turbulence, whether from wings and other flight control surfaces on aircraft, or from any of the other foil-like or bluff bodies.